1. Field of the Invention (Technical Field)
The present invention relates to neutron detectors, more particularly neutron detectors with solid absorbers.
2. Background Art
Neutron detectors are useful in a number of industries, including the oil industry for detecting potential oil yielding sites, the medical field, and surveillance in nuclear facilities and weapons storage. Neutrons are uncharged particles generally contained within the nucleus of an atom, which do not ionize matter as they pass through it. Therefore, unlike their proton and electron counterparts, they are difficult to detect directly. Indirect evidence of a neutron event must be used for detection, instead of detection of the neutron itself.
Neutron detection is an unequivocal signature for the active/passive determination of the presence of transuranic elements associated with nuclear power generated plutonium and declared enriched uranium and plutonium derived from the disassembly of nuclear weapons. Many commercialized neutron detection systems are available in the industry. For example, a neutron detector, as disclosed in U.S. Pat. No. 5,334,840 to Newacheck et al., detects photons of light emitted by carbon infiltrated boron nitride in its hexagonal form when bombarded by neutrons. The amount of light detected correlates to the number of neutrons bombarding the boron nitride.
Another commercially available neutron detector utilizes Helium-3 as the neutron absorber. Helium-3 decomposes into H and H3 when bombarded by neutrons and simultaneously emits electrons with an energy of 764 keV. The ionization of the electrons can be detected using conventional methods well known in the art. This type of neutron detector requires a long collection time for the resulting ionization, requiring integrating and differentiating time constants of between 1 and 5 microseconds for the best results.
Neutron detection for monitoring the dose of thermal neutrons given patients receiving boron neutron-capture therapy utilizes Lithium-6 and a cerium activator in a glass fiber. U.S. Pat. No. 5,973,328 ('328 Patent), which is hereby incorporated by reference, went beyond this prior art technique by allowing a cerium activated glass fiber to be coated with fissionable elements. The '328 Patent first incorporated sol-gel techniques into neutron detection.
The sol-gel area of chemistry, first discovered in the 1800s, received renewed interest when the process was found useful in producing monolithic inorganic gels at low temperatures which could then be converted to glasses without a high temperature melting process. Sol-gel techniques are especially helpful in producing neutron detectors since fissionable materials may be incorporated that would not otherwise survive the high melting temperatures of making glass components.
As discussed above, neutrons are not detected directly. Therefore, emissions detectors such as microchannel plates, channeltrons, or avalanche photodiodes are common in the industry for detection of ultraviolet light and fissioned particles such as electrons. Microchannel plates and channeltrons both operate on the same basic principle of amplifying proportional signals emitted from fissionable materials. A microchannel plate is usually formed lead glass with a uniform porous structure of microchannels. Each functions as a channel electron multiplier, independent of its adjacent counterpart. A thin metal electrode is vacuum deposited on both the input and output surfaces to electrically connect the channels in parallel.
Channeltrons are horn-shaped continuous dynodes, coated on the inside with an electron emissive material. When an ion strikes the channeltron, it creates secondary electrons that have an “avalanche effect,” creating more secondary electrons and ultimately a current pulse.
The '328 Patent discloses sol-gel techniques to provide a device that has a glass film containing a fissionable material such as lithium oxide, uranium oxide, thorium oxide, plutonium oxide, or neptunium oxide which, when bombarded with neutrons, emits a prompt electron, proton, triton, or fission fragment which is detectable by standard UV and particle detectors as discussed above, e.g. microchannel plates or channeltrons. This doped glass film containing a fissionable material can be used in conjunction with a detector component for a useful detection device. The '328 patent also discloses use of a rare earth element, i.e. cerium oxide, that fluoresces when ionized to enhance the UV light emission.
Conventional neutron detection devices typically incorporate exotic materials and are expensive and difficult to maintain, given the sensitivity of the equipment. The '328 Patent device, while certainly a great step forward in ease, accuracy, and affordability of detection, lacks a desired level of sensitivity with respect to distinguishing between neutrons and other radiation fragments such as gamma rays.
The present invention incorporates the effectiveness of the sol-gel chemistry doping techniques found in the '328 Patent combined with emission detectors that have an added distinguishing capability with regard to types of radiation emitted, thereby overcoming some of the disadvantages of the prior art described herein.